The present invention pertains to integrated circuits, and particularly to MOS/LSI circuits.
The generation of valid logic signals in integrated circuits depends in part on the maintenance of power supply voltages within specified limits. In more complex integrated circuits it is important to be able to sense internally whether the external voltage being supplied is sufficient for reliable operation.
In TTL integrated circuit devices, it is a relatively simple task to establish a reference voltage by means of a zener diode, such that the integrated circuit becomes operable when the power supply voltage exceeds the reference voltage. Unfortunately, the zener diode is not available as a practical matter in MOS integrated circuit devices. Therefore, alternative means must be employed in MOS devices for assuring that adequate voltage levels are being supplied.
One technique that is successful in certain MOS integrated circuit applications is the use of delay circuitry that causes the integrated circuit to remain inoperable for some period of time after the onset of power. Such a technique depends for its success on a correct assumption about the rise time of the external power supply, and therefore should be used only in applications where the consequences of an incorrect assumption can be tolerated. For example, some pocket calculators operate on such a delay principle. If the calculator powers up improperly, it can be cleared manually. Such delay circuit techniques, however, are unsatisfactory for integrated circuit devices that cannot tolerate the risk of premature operation that would occur if the power supply failed to reach a sufficient voltage level within the delay time. The microcomputer is one such device.
A single chip MOS/LSI microcomputer is disclosed in commonly assigned U.S. Patent Application Ser. No. 815,932, filed July 15, 1977. Because the microcomputer has such great versatility, it must be able to perform correctly in many different applications, some of which cannot tolerate the generation of improper logic signals under any conditions, including start-up. Furthermore, since the microcomputer operates under the direction of an internally stored program, it is most important that information processing begin at the appropriate starting point in the program's instruction set. It will be appreciated that the generation of false signals during start-up could prevent proper program execution.
Accordingly, the present invention provides internal circuitry for use in MOS/LSI integrated circuit devices, particularly microcomputers, that will give a positive indication that the external power supply has reached a minimum specified voltage level, without regard to rise time.